Initial, experimental proto support in Streamy.

What works:
* Single proto file schema input.
* Strings, int32/int64s, and message types.
* Repeated fields.
* [Un]marshalling by tag number.

What doesn't (including but not limited to):
* Service definitions.
* Imports.
* Enums.
* Libraries.
Right, but this particular piece of API doesn't have access to the Transform instance. And the problem needs to be solved not just for the proto itself, but for any imports it has, too.

This is complicated by the fact that Transform.readInput() takes an AssetId, not a path. The two are not the same, and constructing an AssetId for an arbitrary path is nontrivial. This is a problem I want to tackle in the future, after the prototype works completely.